The present invention relates to a process and apparatus for producing non-oxide compounds. More particularly, invention is concerned with a process and apparatus for continuously producing non-oxide ceramics such as silicon nitride, silicon carbide, boron nitride, aluminum nitride, and sialon.
In general, the production of the above-mentioned non-oxide compounds involves the reaction of raw materials at an elevated temperature of 1300-1800.degree. C. in a non-oxidizing gas such as nitrogen. The following processes have been proposed for the production under such conditions.
(1) A process that employs a single-stage vertical furnace. According to this process, a vertical reaction chamber made of refractories is filled with raw materials and supplied with a non-oxidizing gas. While falling by gravity, the raw materials are heated by heaters attached to the outside of the reaction chamber. (Japanese Patent Laid-Open No. 33899/1979)
(2) A process that employs an externally heated rotary furnace of a dual cylinder structure made up of a carbon cylinder and a protective cylinder to prevent oxidation. A non-oxidizing gas is passed through the dual cylinder while it is externally heated by a fuel oil burner or the like. (Japanese Patent Laid-Open No. 74635/1986)
(3) A process that employs an internally heated rotary furnace of such a structure that a rod heater is arranged in a rotating carbon cylinder covered with an insulating material. In operation, a non-oxidizing gas is passed through the rotating carbon cylinder. (Japanese Patent Publication No. 42000/1979)
(4) A process for the production of silicon nitride that employs a single-stage vertical furnace made up of a carbon reaction vessel. At the top of the reaction vessel is a feed inlet and at the bottom is a porous plate through which a gas is introduced and on the outside is an induction heating coil. (Japanese Patent Laid-Open No. 251108/1985)
The above-mentioned conventional processes, however, have their respective disadvantages. According to process (1), a non-oxidizing gas is passed through a reaction chamber filled with raw materials. This prevents the gas from flowing uniformly through the raw materials, leaving unreacted oxides and forming undesirable intermediates. With this process, it is difficult to produce products of uniform quality.
According to processes (2) and (3), the reaction is performed while raw materials are being stirred. This permits the production of a non-oxide powder of uniform quality. However, when the powder bed is thicker than 50 mm, the gas does not diffuse into the raw materials completely and there is a strong possibility that the raw materials partly remain unreacted. Thus the powder bed is limited in thickness. An additional disadvantage is that these processes require a large-scale apparatus for mass production, which leads to high high investment and operating costs. Moreover, passing an excess of non-oxidizing gas makes these processes unsuitable for industrial production.
In process (4), raw materials are uniformly mixed with a non-oxidizing gas; however in order to fluidize the raw materials a large amount of non-oxidizing gas is required. The use of a large amount of gas causes a partial loss of SiO, an intermediate in the production of silicon nitride as shown in the following equations. This leads to a decreased yield. EQU SiO.sub.2 +C.fwdarw.SiO+CO (1) EQU 3SiO+3C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +3CO (2)
A common disadvantage of the above-mentioned conventional processes is that the reaction gas at a high temperature is discharged as such and consequently a large amount of heat energy is lost. In the case in which a carbon structure is used in the apparatus, the raw materials should be bone dry and an additional apparatus is required to dry the raw materials.